1. Field of the Invention
The present invention relates to an ordinary-temperature purifying catalyst, which can purify and remove an environmental loading material, such as carbon monoxide (CO), odorants, nitrogen oxides (NOx) and hydrocarbons (IC), formaldehyde which often generates in houses, and an ethylene gas which causes to deteriorate the freshness of foods, at an ordinary temperature (e.g., room temperature).
2. Description of the Related Art
Conventionally, in order to remove odorants and harmful gases, adsorbents, such as activated carbon and zeolite, have been used so far. However, the adsorbents require means for processing the adsorbed components.
Further, in order to purify carbon monoxide, one of harmful gases, the performance is required which purifies and removes carbon monoxide in a lower temperature range and in a shorter period of time. However, conventional catalysts have been still insufficient in the purifying ability at an ordinary temperature. Accordingly, a further improvement is required in the purifying performance.
The influences of formaldehyde to human bodies are irritating actions to the eyes, nose and throat. For example, the influences accompany syndromes, such as discomfort feelings, tearing, sneezing, coughing, vomiting and difficult breathing. The sources of the generation in the housing spaces are believed to be cigarette smoking, usages of heating apparatuses, home appliances, pieces of furniture and building materials. The formaldehyde is used as urea-formaldehyde adhesives in a large amount, is a raw material for adhesives for plywoods and particle boards, and is used as antiseptics for adhesives for wall papers. Accordingly, the Japanese Health and Welfare Ministry presents that the reference value of the formaldehyde concentration be 0.1 mg/Nm 3 (=80 ppb) or less in a room.
Recently, the formaldehyde has been regarded as one of the materials which cause the sick-house syndromes. Accordingly, the material makers and the housing makers try to construct houses with materials, which do not generate the formaldehyde, and to age houses so as to reduce the formaldehyde concentration before they hand over the houses to the customers. However, it cannot be said that the aforementioned treatments necessarily satisfy the reference values of the Japanese Health and Welfare Ministry.
Whilst, in order to remove the volatile formaldehyde, adsorbents, such as activated carbon and zeolite, have been used. However, the adsorbents have limited longevity. Accordingly, it is necessary to regularly replace the adsorbents, and to spend economical expenses. Further, it is necessary to provide means for processing the components adsorbed on the adsorbents anew.
As means for removing thee indoor odorants, etc., at an ordinary temperature, air purifiers have been known generally into which activated carbon is incorporated. Further, air purifiers have been commercially available which utilize photocatalysts.
In addition, the following have been put in markets: namely; methods for removing the odorants, etc., with ozone; methods by using photocatalysts therefor; apparatuses therefor; and filters therefor. However, regarding ozone, it is necessary to exceed the regulatory value of the ozone concentration in order to exhibit the advantages. Further, it is necessary to provide a catalyst for removing the ozone. Concerning the photocatalytic system, it is necessary to provide an artificial light source which excites the photocatalysts. Further, it is necessary to spend expenses for electricity when the light source always irradiates the photocatalysts. Hence, the costs go up.
In Japanese Unexamined Patent Publication (KOKAI) No. 6-219,721, a catalyst is disclosed. The catalyst includes metallic oxide particles, in which noble metal particles are mixed uniformly, and which are composed of CeO2. The metallic oxide can be at least one member selected from the group consisting of CeO2, ZrO2, TiO2 and SnO2 This catalyst is made by coprecipitating the metallic oxide, which is composed of the noble metal particles for purifying carbon monoxide to carbon dioxide. The catalyst has anionic holes in the surface so that it exhibits a high reactivity without a hydrogen reduction treatment. However, the publication discloses that the purifying performance was evaluated under the temperature condition of 150xc2x0 C. or more, and does not refer to the purifying activity in a room temperature range.
Whilst, a specific catalyst is proposed as a method for oxidizing and decomposing odorants or harmful gases in air. For instance, Japanese Unexamined Patent Publication (KOKAI) No. 10-296,087 discloses that a catalyst,, which requires zirconia or ceria as an indispensable component and at least one member selected from the group consisting of Ag, Pd, Pt, Mn and Rh is loaded thereon, can be used to oxidize and decompose trimethylamine. However, in this case as well, the disclosed catalyst was simply examined for the purifying activity under the temperature condition of 200xc2x0 C. in the examples. Further, the publication does not set forth the oxygen defect of the oxide used in the catalyst. Therefore, it is not believed that the catalyst keeps a sufficient ability of decomposing trimethylamine at a room temperature.
In a publication of the PCT applications, e.g., No. WO-91/01,175, a catalyst is disclosed which oxidizes oxydants in a low temperature range. The catalyst includes at least one reducing metallic oxide selected from the group consisting of iron oxide, ceria, zirconia, copper oxide, rare-earth element oxide, manganese oxide, vanadium oxide and chromium oxide, and a noble metal loaded on the metallic oxide. According to the publication, the catalyst oxidizes oxydants whose molecular weight is 50 or less at a temperature of up to 30xc2x0 C. When paying attention to the reduction temperature as one of the production conditions of this catalyst, the publication sets forth that the reduction treatment was carried out at room temperature. However, it is impossible to introduce an oxygen defect, which exhibits a function of the present invention, into the aforementioned metallic oxides at room temperature.
In Japanese Unexamined Patent Publication (KOKAI) No. 7-51,567, a catalyst is disclosed. The catalyst is made by suspending activated carbon in an aqueous solution, in which molybdenum or cerium and platinum are solved, and by loading the molybdenum or cerium and the platinum on the activated carbon with heat or a reducing agent. However, the publication does not refer to subjecting the catalyst to a certain treatment after the loading.
Generally speaking, air cleaner filters using activated carbon effect the deodoring ability by physically adsorbing the odorants on the activated carbon. However, the activated carbon exhibits an upper limit of the adsorption, and does not effect the deodoring performance more than the saturated adsorption. Accordingly, the filters have short lives. Hence, it is essential to regenerate the filters for a long period of usage. When the regenerating is not carried out, it is indispensable to replace the filters.
In the filters using photocatalysts, it is indispensable to irradiate a light to actuate the photocatalysts in order to purify harmful substances. In order to carry out this, it is necessary to use a light source, such as a mercury lamp. Therefore, there arises a drawback in that the air cleaners, which employ the filters using the photocatalysts, are remarkably expensive in terms of the equipment costs and the running costs.
An ethylene gas contained in air is believed to degrade the freshness of fruit and vegetables by facilitating the physiological actions of fruit and vegetables and by developing the additional aging thereof. Accordingly, in order to preserve the freshness of perishables, the following methods are proposed: namely; decomposing the ethylene with ozone or hydrogen peroxide; and adsorbing the ethylene.
However, in the aforementioned methods, the adsorbing method exhibits the shortened duration of the good effects. Accordingly, when the ozone decomposition or the photocatalyst is used together with the adsorbing method in order to compensate for the disadvantage, there arises a problem in that an apparatus taking part in the removing means is large-sized so that it is too costly or it requires a large equipment space. Thus, the adsorbing method has a drawback in terms of the practical application.
The present invention has been developed in view of the aforementioned circumstances. It is therefore an object of the present invention to provide an ordinary-temperature catalyst which can decompose or purify the environmental loading material, such as formaldehyde and ethylene, contained in air at an ordinary temperature, and which is less likely to degrade chronologically. The xe2x80x9cordinary temperaturexe2x80x9d hereinafter means a temperature range including a room temperature which does not require special heating and cooling or less.
An ordinary-temperature purifying catalyst according to the present invention comprises: an oxide having an oxygen defect introduced by a reduction treatment; and a noble metal loaded on the oxide. By loading the noble metal on the oxide having an oxygen defect, the present catalyst has an ability of purifying an environmental loading material in air at an ordinary temperature. As for the environmental loading material, it is possible to list odorants, carbon monoxide, hydrocarbons, such as ethylene, and nitrogen oxides.
The oxide constituting the present ordinary-temperature purifying catalyst can preferably be at least one oxide selected from the group consisting of an oxide of transition metal elements and an oxide of rare-earth elements. As for the oxide of transition metal elements, it is possible to use at least one oxide selected from the group consisting of oxides of zirconium, iron, manganese, cobalt, nickel, copper, chromium, molybdenum and niobium. As for the oxide of rare-earth elements, it is possible to use at least one oxide selected from the group consisting of oxides of cerium, yttrium, neodymium, praseodymium and samarium.
The oxide constituting the present ordinary-temperature purifying catalyst can be composed of a cerium oxide, and at least a part of the cerium oxide can have an oxygen defect by a reduction treatment. The cerium oxide can preferably be represented by a formula of CeOn wherein 1.5xe2x89xa6n less than 2. The cerium oxide can further preferably be represented by a formula of CeOn wherein 1.5xe2x89xa6nxe2x89xa61.8. The present catalyst including the cerium oxide is capable of purifying at least one member selected from the group consisting of aldehyde, amine, mercaptan, fatty acids, aromatic hydrocarbon, and odorants.
In the present ordinary-temperature purifying catalyst, the oxide can preferably include a cerium oxide and a zirconium oxide, at least a part of the cerium oxide having the oxygen defect. Further, the cerium oxide and the zirconium oxide can preferably form a solid solution, a composite oxide, or a mixture thereof.
The oxide can preferably have a specific surface area of 50 m2/g or more by being subjected to a reduction treatment at 500xc2x0 C. for 1 hour. Under different conditions, the oxide can preferably have a specific surface area of 15 m2/g or more by being subjected to a reduction treatment at 800xc2x0 C. for 1 hour.
The temperature of the reduction treatment can preferably fall in a range of from 100 to 800xc2x0 C., further preferably in a range of from 200 to 600xc2x0 C.
The present ordinary-temperature purifying catalyst can be loaded on at least one support selected from the group consisting of titanium oxide, alumina, silica, zeolite, cordierite, sepiolite and activated carbon.
After a purifying reaction of the odorant is carried out at 25xc2x0 C. at a space velocity of 600,000 hrxe2x88x921, the present ordinary-temperature purifying catalyst can have a conversion ratio of 50% or more at 20 minutes after starting the purifying reaction. Further, the present ordinary-temperature purifying catalyst can have a conversion ratio of 40% or more at 30 minutes after the initiation of the purifying reaction provided that the carbon monoxide is supplied at 25xc2x0 C. at a space velocity of 120,000 hrxe2x88x921.
A method for using an ordinary-temperature purifying catalyst according to the present invention comprises a step of contacting the above-described ordinary-temperature purifying catalyst with air including at least one environmental loading material selected from the group consisting of carbon monoxide, a nitrogen oxide, ethylene, aldehyde, amine, mercaptan, fatty acids, aromatic hydrocarbon, and odorants, thereby purifying the environmental loading material at an ordinary temperature.
Since the present ordinary-temperature purifying catalyst comprises the noble metal loaded on the oxide at least a part of which has an oxygen defect by a reduction treatment, the harmful materials. can be removed and purified by the actions of the both of the oxide and the noble metal. Particularly, it is possible to purify even the harmful materials which exist in a trace amount in air. Especially, when the present ordinary-temperature catalyst includes the cerium oxide and the noble metal, it is possible to efficiently remove the formaldehyde in air at room temperature. Thus, the present ordinary-temperature catalyst can reduce the loads to human bodies in the housing environment, and contribute to improving the housing environment.
Further, since the present ordinary-temperature catalyst can decompose and remove the ethylene in an ordinary temperature range, it is possible to suppress the physiological actions of fruit and vegetables so as to inhibit them from chronologically aging additionally. Thus, it is possible to easily keep the freshness of fruit and vegetables.
Furthermore, since the present ordinary-temperature catalyst carries out an oxidation reaction, it yields carbon dioxide and consumes oxygen while it decomposes the ethylene. Accordingly, it is possible to form an optimum gas composition for keeping the freshness of fruit and vegetables whose oxygen concentration is low, carbon dioxide concentration is high and ethylene concentration is low.
Moreover, when the present ordinary-temperature catalyst comprises the activated carbon on which the cerium oxide with an oxygen defect and the noble metal are loaded, the adsorbing ability of the activated carbon can be utilized. Accordingly, it is possible to reduce the amount of the harmful materials which contact with the noble metal and the cerium oxide in a unit period of time. Thus, the catalytically active portions can be inhibited from being poisoned by the adsorbed harmful materials. As a result, it is possible to maintain the purifying performance for a long period of time.